Magnesium bimetal and system for flame spraying metals on magnesium substrate

ABSTRACT

An oxyacetylic process for depositing and bonding a layer of other metal onto a sheet of magnesium without igniting the magnesium by flame spraying using a flame spray gun.

United States Patent 1 1 3,631,835

[72] Inventors l-lugh C. l-lamontre [50] Field of Search 1 17/1052; Riverside; 29/196.2, 197, 197.5, 198; 279/3; 248/362; Hugh M. D. Kessler, Corona, both of Calif. 118/47, 50, 59, 69, 269; 219/76; 269/21 [211 App]. No. 802,526 [22] Filed Feb. 26, 1969 References Cited [45] Patented Jan. 4, 1972 UNITED STATES PATENTS 1 Assignee The United States of America 88 3,077,674 2/1963 Mueller 248/362 x "Presented by the Secretary of Navy 3,135,626 6/1964 M0611 6t a1 1 18/47 x Primary Examiner-Ralph S. Kendall [54] MAGNESIUM BIMETAL AND SYSTEM FOR Assistant ExaminerEdward G. Whitby FLAME SPRAYING METALS ON MAGNESIUM AttorneysR. S. Sciascia and J. M. St. Amand SUBSTRATE 1 Claim, 3 Drawing Figs.

ABSTRACT: An oxyacetylic process for depositing and bond- [52] ing a layer of other metal onto a sheet of magnesium without 7 1 "'t'th 'bfl *"fl' 29/196.2, 29/197, 29/198, 29/199, 269/21, 219/76 53 e magneswm y me Sp'aymg pray [51] Int. Cl ..B05c ll/l2, 305C 1 H00 FLAME SPRAYING METALIZING GUN f/l/M 11 20 MAGNESIUM SUBSTRATE AIR BAFFLE AIR INLET 2 mzmmm m 3631.835

FLAME SPRAYING METALIZING GUN AIR BAFFLE AIR INLET I 2 MAGNESIUM METAL COATING |6'-\ FIG. 3

A I g M GNES UM y \r/ HUGH c. HAMONTRE Z HUGH M. D. KEESLER 5 Z INVENTORS IO 24 4 FIG. 2

4 BY W4 W 7/ ATTORNEY MAGNESIUM BIMETAL AND SYSTEM FOR FLAME SPRAYING METALS N MAGNESIUM SUBSTRATE The invention herein described may be manufactured and used by or for the Government of the United States without the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION The instant invention is for a process whereby other metals can be deposited on magnesium by flame spraying without igniting the magnesium substance. Copper, molybdenum, nickel, and stainless steel have been successfully deposited on magnesium by the instant process to form a bimetal in each instance. Other metals that can be flame sprayed, using the oxyacetylic process, can be deposited by the same method and the process is not limited to the above-named metals.

The process described herein provides a means whereby a bimetal of any thickness can be fabricated and the thickness of either, or both, of the metals can be controlled. No special handling is required, and standard commercial oxyacetylene flame spray guns are used.

The magnesium bimetal described herein has been developed particularly for use as the anode-cathector assembly in high-energy nonaqueous batteries. It consists of a sheet of magnesium metal on which has been bonded a second metal by oxyacetylene flame spraying. Such a combination provides a bimetal of the necessary cross-sectional thickness, and chemical and metallurgical characteristics required for the proper functioning of an anode-cathector assembly in a battery. In addition, such sprayed panels can be flame sprayed to any desired thickness and used in any structure as desired. Proper selection of the sprayed material can provide protection of magnesium from corrosion by metallurgically sealing the exposed surfaces of the magnesium from a corrosive environment. Furthermore, such a sprayed magnesium surface will provide a significant reduction of fire hazard in magnesium structures of all types.

Prior to the present process and the development of flame sprayed magnesium, limited quantities of bimetal have been available from only two sources. One consisted of two sheets of material, one of which was magnesium spotwelded at numerous points over the surface of the sheets being formed. The second was a cold-welded sheet formed by stacking two sheets and pressure cold-welding them by passage through a heavy rolling mill.

In the instance of spotwelding, it was obvious that contact did not occur over the total surface and the electrolyte in the battery penetrated between the two sheets of metal. In battery reactions it is essential that electrical contact be made on the total surface between the two metals because as the magnesium is used in the reaction, the electron collector metal must remain in contact with the magnesium or the cell is disabled by loss of electronic conductance into the external circuit. In addition, the spotwelded areas separate when attacked by the electrolyte, thus breaking electrical conductance. The presence of the electrolyte between the two sheets also causes the generation of heat by side reactions and the evolution of gas between the anode (magnesium layer) and the collector (second metal layer), both of which further reduce the electrical conductivity in the battery cell. This, it is demonstrated that a spotwelded bimetal cannot be used in the production of high-efficiency battery cells.

The cold-welded bimetal that has been used for battery cathodes is very difficult to find commercially and is extremely costly. In addition, it is not available in thicknesses required for many applications. The commercial source for such material only produces a sheet of 0.015 inch minimum thickness, which doubles the ratio of cell volume to output rates. For high-efficiency reserve batteries, increased volume is a significant factor in the design. Furthermore, only one combination of bimetal is available, thus limiting the possible choices of materials in battery design.

Heretofore, the ability to apply satisfactory metallic coatings on a magnesium substrate have not been successful,

and the inverse process of coating magnesium on other metals have not been very successful.

DESCRIPTION OF THE DRAWINGS Other objects and many of the attendant advantages of this invention will become readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

FIG. 1 shows a cross-sectional view of a fixture for holding and cooling a sheet of magnesium to be flame sprayed with another metal;

FIG. 2 is a cross section taken along line 2-2 of FIG. 1; and

FIG. 3 is a cross-sectional view of a sheet of magnesium bimetal.

DESCRIPTION OF THE INVENTION The process of the instant invention is best described by referring to FIG. 1 of the drawing which shows a fixture used for holding and cooling a magnesium part to be flame sprayed. This holding fixture assembly operates to maintain the magnesium below its ignition point while it is being flame sprayed with another metal using oxyacetylene metal spray gun. The fixture consists of a steel tube 10, square in cross section, having an adjustable gate 12 at one end that controls the flow of air through aperture 14 into the fixture. At the opposite end of tube 10 is a round tubular opening 15 to which is connected a low-vacuum, high-volume pump. A short cylindrical tube 16 is mounted in an aperture in one face of rectangular tube 10, ash shown in FIG. 1. The end of tube 16 is closed by a flat plate 18 having a series of small holes 19 therein. Holes 19 allow vacuum to be applied to a disk 20 of magnesium, thus holding it in place while a second metal is applied to the other surface by means of an oxyacetylene metal spray gun 22. Commercially available metal spray guns, such as a Metco type 4E metallizing spray gun, for example, can be used to flame spray and deposit various types of metals onto the magnesium substrate 20 while it is maintained below its ignition point by the vacuum-operated holding fixture assembly.

A baffle 24 is located in square tube 10 to deflect air flowing through aperture 14 into the cylindrical chamber of tube 16 against the sidewall of tube 16 at its midpoint 26. By so directing the airflow, an eddy current is generated in the cylindrical chamber of tube 16 directly under plate 18 which supports the magnesium part 20 being flame sprayed. The airflow pattern described above maintains cool air on plate 18 and also on magnesium part 20 via holes 19 to prevent heating of the magnesium part to the ignition point while at the same time the vacuum holds part 20 in place against plate 18. By the combination of vacuum and directed airflow, magnesium can be flame sprayed without burning during the process.

The system as described above requires a rather precise balance between airflow through the system and the degree of vacuum. This ratio of flow to vacuum is achieved by adjustment of gate 12 over aperture 14 until the flow of air striking baffle 24 is sufficient to generate the required eddy flow in the camber of cylindrical tube 16. This is a readily made adjustment and need only be made once for a given source of vacuum.

As the fixture is used, a vacuum pump 25 is connected to tubular opening 15 located in the opposite end of the square tube 10 from the gate 12 and aperture 14, thus creating a unidirectional flow through the fixture. By this means, a supply of cool air is provided for the proper ratio of flow to vacuum required for holding the magnesium part in place and maintaining its temperature below the ignition point.

No other method of flame spraying magnesium metal is known and the method described herein is unique. It provides a means for the production of a bimetal combination with magnesium requiring no special treatment of the metals involved, may be performed without the use of an inert atmosphere, and utilizes readily available materials and equipment; thus, it is economical and provides a choice of many metal combinations with magnesium that, heretofore, have not been available.

The process was developed primarily for use in the development of high-energy batteries, but can be used for the protection of magnesium bodies of all types for corrosion, and for deposition of fire retardent coatings on magnesium to protect it from ignition; thus making utilization of the high strength to weight ratio of the magnesium under conditions not heretofore possible.

The flame-sprayed bimetal described herein consists of a layer of magnesium metal on which has been flame sprayed a layer of another metal to any desired thickness. The magnesium metal can be cleaned, for example, by either of two methods: one by etching for seconds in a dilute solution of sulphuric acid containing approximately 2 percent by volume of concentrated sulphuric acid in water; this treatment removed the heavy oxide coating from the magnesium and,

after rinsing the etched surface in acetone and drying in a warm airstream, the magnesium was stored in a container until ready for flame spraying. The second method uses mechanical cleaning with an abrasive paper to remove the oxides from the surface of the magnesium metal. Either of the two methods are satisfactory and the resulting cleaned magnesium metal can be stored for several weeks before flame spraying with the second metal. immediately prior to flame spraying, the magnesium was scoured with a medium grade of steel wool that had been thoroughly washed in acetone. It then was placed on the holding fixture previously described, and flame sprayed to the thickness desired with the second metal.

Sheets of bimetal are produced by the system disclosed having continuous electrical contact and physical bonding over the entire interface of the two metals. This prevents penetration of electrolyte between the metals. Thus, all of the undesirable features of spotwelded bimetal were overcome. In addition, sheets of bimetal can be produced as thin as 0.008- inch thick, thus reducing the bulk factor by two. The total thickness of the bimetal sheet is determined by the thickness of the substrate (magnesium sheet) and the amount of the second metal flame sprayed on it. Magnesium sheet 0.005 inch thick, for example, has been used and at least 0.004 inch of a second metal (aluminum, copper, 316 stainless steel, molybdenum, and nickel, for example) have been deposited on the magnesium substrate to form a proper liquid seal. Magnesium foils of less than 0.005 inch thickness can be sprayed, and other metals may be used on magnesium, thus increasing further the cell efficiency with respect to thickness, and further increase the energy volume ratio.

What is claimed is:

1. An apparatus for making flame-sprayed magnesium bimetal having continuous electrical contact and physical bonding over the entire interface between the magnesium and flame-sprayed metal coating, comprising:

(a) a flame-spray metallizing gun,

(b) a vacuum assembly for holding and cooling a cleaned sheet of magnesium to be coated while said sheet is being coated with another metal by means of said metallizing gun,

(c) said vacuum assembly, comprising 1. a vacuum chamber having a controlled air inlet aperture at one end and na outlet connected to a vacuum pump at the opposite end to create a low vacuum and high volume of airflow therein, said controlled air inlet aperture operable to vary the ratio of the degree of airflow through the system and degree of vacuum as desired,

2. an eddy current chamber extending from one side of said vacuum chamber between the opposite ends thereof and being open to said vacuum chamber,

3. an outer wall of said eddy current chamber being a flat surface having a plurality of relatively small holes therethrough whereby vacuum in said chambers is applied to the sheet of magnesium to be coated via the plurality of holes in said eddy current chamber to hold the sheet of magnesium in place on the outer wall flat surface of said eddy current chamber over said plurality of small holes, while being coated,

4. a baffle means located in said vacuum chamber for deflecting into said eddy current chamber air entering said vacuum chamber via said controlled air inlet to create an eddy current flow of air in said eddy current chamber for cooling and maintaining the magnesium sheet at a temperature below its ignition point while being coated with another metal by said flame-spray metallizing gun. 

2. an eddy current chamber extending from one side of said vacuum chamber between the opposite ends thereof and being open to said vacuum chamber,
 3. an outer wall of said eddy current chamber being a flat surface having a plurality of relatively small holes therethrough whereby vacuum in said chambers is applied to the sheet of magnesium to be coated via the plurality of holes in said eddy current chamber to hold the sheet of magnesium in place on the outer wall flat surface of said eddy current chamber over said plurality of small holes, while being coated,
 4. a baffle means located in said vacuum chamber for deflecting into said eddy current chamber air entering said vacuum chamber via said controlled air inlet to create an eddy current flow of air in said eddy current chamber for cooling and maintaining the magnesium sheet at a temperature below its ignition point while being coated with another metal by said flame spray metallizing gun. 